Carpet Underlay Explained: How to Choose the Right Underlay for Your Home

Underlay is the part of a new carpet that nobody sees, and it does more for comfort, warmth, noise and lifespan than most people expect. A good carpet on poor underlay will wear out sooner and feel thinner than it should, while a sensible mid-range carpet on the right underlay can feel far more luxurious. Here is how to choose well, from a family-run carpet showroom that has been fitting carpet across London for over 25 years.

What underlay actually does

Underlay sits between the floor and the carpet and does four jobs. It absorbs the impact of footsteps, which protects the carpet pile from crushing. It adds insulation and comfort underfoot. It reduces noise, both inside the room and through the floor to the room or flat below. And it evens out minor imperfections in the subfloor, so they do not show through the carpet over time.

The main types of carpet underlay

PU foam

Polyurethane (PU) foam underlay is made from recycled foam bonded together. It is soft and comfortable, offers good insulation, and is the most common choice for bedrooms and living rooms. Denser grades last longer and feel more supportive, so it is worth looking at density as well as thickness.

Crumb rubber

Crumb rubber is made from recycled rubber and is very dense and hard-wearing. It feels firm rather than soft, which makes it a good choice for hallways, stairs and other busy areas where support matters more than a cushioned feel.

Sponge rubber

Sponge rubber underlay usually has a waffle pattern on the underside. It is springy and comfortable and works well in most rooms of a home, although it can be heavier and more expensive than foam.

Felt and combination underlays

Felt underlays, often made from recycled fibres, are dense and stable. They are a traditional choice under heavier carpets and on stairs. Some modern underlays combine felt with rubber or foam to get the stability of one with the comfort of the other.

Choosing underlay room by room

• Bedrooms: comfort comes first, so a good-quality PU foam or sponge rubber underlay is usually ideal.
• Living rooms: moderate traffic and furniture, so choose a denser foam or a combination underlay that will not compress under sofas.
• Hallways and landings: heavy traffic, so prioritise density and support with crumb rubber or a dense combination product.
• Stairs: use a firm, stable underlay. Soft, thick underlays let carpet move on the nosing of each step, which causes early wear and can be less safe.

Underlay and underfloor heating

Carpet works well with underfloor heating as long as heat can pass through it. Insulation is measured in tog, and the combined tog of the carpet and the underlay should not exceed 2.5 tog. For systems run by a heat pump, which operate at lower water temperatures, aiming for 1.5 tog or less is better.

In practice this means choosing an underlay made specifically for underfloor heating, which is typically thin and dense, and checking the tog of the carpet you have chosen. A thick, high-tog underlay can trap heat, make the system slow to respond and raise running costs. If you are unsure, tell us the type of heating when we measure, and we will recommend a combination that works.

Underlay for flats and noise

In London flats, noise between floors is one of the most common causes of complaints between neighbours. Carpet on a dense underlay is one of the quietest floor finishes you can choose. Many leases, particularly in mansion blocks, also include clauses about flooring and sound insulation, and some require landlord or managing agent consent before work starts. It is worth checking your lease before you buy. For more on this, see our guide to the best flooring for London flats.

Should you reuse your old underlay?

It is tempting to save money by keeping existing underlay, but it is rarely a good idea. Old underlay is flattened along the routes people walk, and a new carpet laid over it will wear unevenly in those areas. Some carpet manufacturers also require new underlay as a condition of their warranty. Replacing underlay costs far less than replacing a carpet early.

Getting it right

The best underlay depends on the carpet, the room, the subfloor and whether there is underfloor heating. When we visit to measure, we check all of these and recommend the underlay we would choose for our own home, at the price point that suits you. Visit our showroom at 31 Crawford Street, Marylebone, call 020 7224 8876, or book a free home consultation. You can also browse our carpet ranges.